The *Ceasar cipher* is a [[Symmetric Encryption|symmetric encryption algorithm]], i.e. it uses the same key for encryption and decryption. The main mechanism of the algorithm is to shift every letter of the alphabet by a constant amount. If the key is $1$, we shift every letter of the alphabet by one position: $A\rightarrow B$, $B\rightarrow C$, ..., $Z\rightarrow A$. The word *HELLO* becomes *IFMMP* after applying the Ceasar chiffre with the key $1$. While the algorithm is quite simple, this simplicity leads to a series of weaknesses: - There are only 25 different keys, since the alphabet has 26 letters. Shifting by 26 maps each letter to itself and shifting by more than $26$ is equivalent to a key that is already covered by a smaller key. - Languages have characteristic statistics for each letter, e.g. the most common letter in the English language is 'e', followed by 'n' on the second place. If we find the most common letter in a sufficiently long encrypted message, we can easily discern the key by just looking for the most common letter and identifying it with 'e'. This attack is called frequency analysis. A good encryption algorithm obfuscates the frequency of letters. In summary, the *Ceasar cipher* is still applied is instructive as an easy encryption algorithm, but should *never* be used in practice where security is relevant. A natural extension of the *Ceasar cipher* is the [[Vignere Cipher|Vignère cipher]], where each letter is shifted according to a key instead of a single number as in the *Ceasar chiffre*. The cipher only becomes secure in the form of the Vernam cipher, where a [[One-Time Pad|one-time pad]] is used as a key. The main problem of [[Symmetric Encryption|symmetric encryption]] algorithms is the exchange of the key. Traditionally, this problem has been solved with [[Asymmetric Encryption|asymmetric encryption schemes]], where both keys are public. Since these become vulnerable by [[Quantum Computer|quantum computers]], quantum protocols like [[Quantum Key Distribution|quantum key distribution]] are actively studied nowadays. >[!read]- Further Reading >- [[Encryption]] >- [[Symmetric Encryption]] >- [[Encryption Key]] >[!ref]- References